An activity monitor allows the measurement of human energy expenditure. The activity monitor allows the user wearing the activity monitor to measure the amount of energy consumed during a selected period of time for a certain physical activity, such as walking and running. An activity monitor utilizing a single accelerometer is less expensive and consumes less power than an activity monitor utilizing a plurality of accelerometers. A single accelerometer may be a uni-axial accelerometer that generates a signal, which is proportional to the energy expenditure, such that the signal is generated in response to body movements in the particular direction along the single axis of the accelerometer.
When user is performing a low intensity physical activity, such as walking, the dominant direction of body movement is the up and down direction occurring along the vertical axis. If the accelerometer is properly aligned with the body movement of user to measure movements along the vertical axis, the activity monitor will provide a signal, which is a relatively accurate representation of the energy expenditure by the user. However, when user is performing a high intensity physical activity, such as running, the forward and backward movement of the body provides an additional, non-negligible contribution to energy expended by the user. In response to the backward and forward movement occurring along the horizontal axis during high intensity physical activity, the accelerometer will provide a signal, which is an inaccurate representation of the actual energy expenditure by the user because the accelerometer primarily measures movements along the vertical axis. Therefore, the energy expenditure measured by the uni-axial accelerometer in the activity monitor will deviate from the real energy expenditure of the user. One way to solve this problem for accurately measuring energy expended requires using an additional accelerometer, which can measure forward and backward body movement along the horizontal axis. However, this additional sensor will increase the cost of the activity monitor, and the amount of power consumed by the activity monitor.
Additionally, an activity monitor with only one sensor, such as a uni-axial accelerometer, does not include hardware operable to calculate the speed of a user while the user is walking or running. One way to solve this problem for determining the speed of the user requires using an additional sensor for tracking speed, such as an accelerometer worn on the foot or shoe, a switch on the sole of the shoe, or a GPS sensor. However, this additional sensor will increase the cost of the activity monitor, and the amount of power consumed by the activity monitor.